The goal of this proposal is to understand the role of early growth response (Egr) transcription factors in T lineage commitment. The commitment of a multipotential precursor to a single cell lineage is a fundamental process common to development of multicellular organisms, yet the developmental cues that regulate lineage choices remain poorly understood. T Lymphocytes comprise two distinct lineages that express either alpha/beta or gamma/delta T cell receptor (TCR) complexes and perform vital, non-overlapping roles in immune responses. The alpha/beta and gamma/delta lineages arise from a common precursor, but despite much effort, very little is known about the developmental cues that determine which of these lineages a developing thymocyte will adopt. Importantly, we have demonstrated that Egr proteins are superinduced in cells committing to the gamma/delta lineage and that overexpression of Egrl in fetal thymocytes actually promotes development of gamma/delta lineage cells. Since Egr proteins have been shown to be induced in proportion to signal strength in a variety of different cellular contexts, we now advance the signal strength model as an explanation for how alpha/beta-gamma/delta lineage commitment is regulated. The signal strength model proposes that irrespective of the TCR from which the signal is transduced, strong TCR signals favor selection of the gamma/delta fate, whereas weak signals favor the alpha-beta lineage. Moreover, we propose that the deterministic differences in signal strength are manifested by proportional induction of Egr proteins. We intend to test our hypothesis as follows. In Aim 1 we will investigate the mechanistic basis for promotion of gamma-delta development by enforced expression of Egr proteins. In Aim 2, we will assess the role of E proteins and other Egr-targets in promotion of gamma/delta development. Finally, in Aim 3 we will determine whether the extent of Egr-induction is predictive of lineage fate; and whether induction of Egr proteins in proportion to signal strength is required for lineage commitment to occur with fidelity. In alI of these aims, we wiII exploit our expertise with retroviraI transduction to regulate expression/activity of Egr proteins and their targets in developing thymocytes. Our investigation of the role of Egr proteins in T lineage commitment is of fundamental importance not only for thymocyte development, but also for other developmental processes, since control of cell growth and differentiation is a recurring theme in development and transformation.